Nanostructured materials are of considerable interest due to their unique mechanical properties and structural versatility. Materials with grain sizes less than one micrometer have been shown to have significantly improved mechanical properties compared to corresponding coarse-grained materials under certain conditions. However, the structure of the starting materials, physical treatments, and fabrication conditions can significantly impact the performance of nanostructured materials for specific applications.
Nanostructured materials with high yield strength, hardness, and superplasticity have previously been fabricated. However, poor ductility was observed to accompany these mechanical characteristics especially in high-strength intermetallic compounds. Previously, available nanostructured intermetallics failed in the elastic regime under tensile stresses with virtually no plastic strain-to-failure at room temperature, severely limiting their use in industrial applications. The observed extreme brittleness in nanostructured materials, in particular intermetallics, is attributed to flaws or porosity produced during the fabrication process.
Fabrication of nanostructured materials commonly followed a xe2x80x9ctwo-stepxe2x80x9d consolidation method, which involves synthesizing various powders of nanometer size and then consolidating them into bulk articles using such processes as hot pressing. However, the xe2x80x9ctwo stepxe2x80x9d consolidation processes cannot prevent the formation of micro-flaws or porosity in the final products.
xe2x80x9cOne stepxe2x80x9d methods of nanostructured synthesis (e.g., electro-deposition, crystallization of amorphous solids, and severe plastic deformation) produce materials without residual porosity, but have several disadvantages. First, nanostructured intermetallics made by these methods are extremely brittle. Second, it is difficult to electro-deposit bulk nanostructured intermetallics because of the accumulation of deposition stresses. Thus, known one-step methods of nanostructured synthesis fail to produce materials having both high tensile strength and ductility.
The problem of poor ductility in nanostructured materials is widely recognized in the scientific community. For example, the highest reported strength for nanostructured FeAl intermetallic was found to be 2.3 GPa. However, the material exhibited such poor ductility that the strength was only measurable under compression. In addition, forming bulk amorphous solids is technically complex and not practical for single-phase metallic materials. Single phase solids can be simpler to make, more stable, and may be desirable due to their magnetic, electrical, or optical properties. However single-phase intermetallics have not shown a combination of high strength and good ductility in tension.
Decreasing the grain size is important for increasing strength, but grain size should be decreased while reducing or eliminating the flaws (cracks) and porosity in the materials. Achieving fine grain sizes using severe plastic deformation involving enormous strains by torsion of several hundred percent has met with very limited success in the improvement of tensile ductility. For instance, heterogeneous strain of xcx9c400% at 200xc2x0 C., followed by homogeneous strain of xcx9c800% at 400xc2x0 C., and by additional strain of xcx9c400% at 200xc2x0 C., produces grain sizes of only approximately 1.2 micrometers for Alxe2x80x94Mgxe2x80x94Lixe2x80x94Zr alloys.
Tempering can be used to enhance the toughness of a hardened martensitic phase by converting the metastable martensite to a structure of fine carbide particles in ferrite. However, the tempering process results in materials with enhanced hardness but low ductility.
Preferred embodiments of the invention provide new nanostructured materials and methods for preparing nanostructured materials having increased tensile strength and ductility, increased hardness, and very fine grain sizes making such materials useful for a variety of applications such as rotors, electric generators, magnetic bearings, aerospace and many other structural and nonstructural applications. The preferred nanostructured materials have tensile yield strengths from at least about 1.5 to about 2.3 GPa and a tensile ductility from at least 1%.
Preferred embodiments of the invention also provide a method of making a nanostructured material comprising melting a metallic material into a liquid state, solidifying the material, deforming the material, forming a plurality of dislocation cell structures, annealing the deformed material at a temperature from about 0.30 to about 0.70 of the material""s absolute melting temperature, and cooling the material.
Advantages of the invention will be set forth in part in the description that follows, and in part will be obvious from the description, or may be learned through the practice of the invention. The advantages of the invention will be attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.